The typical dry kit is one of the most simple kits available. The kit is called "dry" because it injects only nitrous into the engine, and not fuel. Generally, the dry kit is for people who want a set that allows them to let the ECU or computer automatically add appropriate amounts of fuel to compensate for the added nitrous into the motor. Usually you can get away with a dry nitrous system if you are confident in your car or truck automatically compensating fuel, you have a larger fuel system, or have a way of ensuring there is adequate fuel when the nitrous gets injected.
The typical wet kit is only of the more popular nitrous kits amongst enthusiasts. The wet kit injects both nitrous and fuel into the engine at the same time. As with the dry kit, you still need to have enough fuel in your fuel system for the wet nitrous system to be able to use while the nitrous is being injected. You can setup an external fuel pump specifically for this kit, so when the nitrous is activated, the fuel will come with the external fuel pump, bypassing the stock fuel system. This is a more elaborate wet nitrous system, but can be one of the most reliable setups.
There are different theories on which nitrous kit is better, but really it depends on what you want, your current setup, and your budget. We have recommended, and will continue to recommend wet nitrous kits. With a proper setup, you can enjoy nitrous shots of 50 to 100hp.